Laboratory Definitions of Performance
From a definition standpoint1,2:
Accuracy (or Trueness) is the closeness of agreement between the average value obtained from a large series of test results and an accepted reference value.
Precision expresses the closeness of agreement (degree of scatter) between a series of measurements from multiple samplings of the same homogeneous sample under the prescribed conditions. Precision may be considered at three levels:
· Within-run Precision (or Repeatability) is the closeness of agreement between results of successive measurements of the same measurand carried out under the same conditions of measurement.
Precision is usually expressed as the Variance, Standard Deviation, or Coefficient of Variation of a series of measurements.
Linearity is the ability to obtain test results directly proportional to the concentration (amount) of analyte in the sample. In other words, a linear device provides a percent change in output equal to the percent change in input.
Reportable Range is the range of values for which the acceptability criteria for the method have been met; that is, where errors due to nonlinearity, imprecision, or other sources are within defined limits.
Reference Range is the central 95% interval bounded by the 2.5 and 97.5 percentiles of the selected sample population.
Once a laboratory has conducted the appropriate protocols and acquired sufficient data to establish their levels of accuracy and precision for a method, they must compare their results to the manufacturer’s published specifications for that method to verify the lab’s qualification to report actual patient results.
1User Demonstration of Performance for Precision and Accuracy: Approved Guideline, EP15-A, NCCLS, December 2001
2 Text on Validation of Analytical Procedures, ICH Harmonized Tripartite Guidelines 27, Oct. 1994